1. Field of the Invention
The present invention relates generally to a piezoelectric transformer, and more specifically to a voltage step-up type piezoelectric transformer which features an improved electrode arrangement which reduces the size of the transformer and ensures highly reliable operation thereof.
2. Description of Related Art
In most of cases, an electromagnetic type transformer is used for generating a high output voltage, of the nature used in photocopiers, backlights of LCD (Liquid Crystal Displays) or the like.
On the other hand, a piezoelectric transformer which operates on a principle which is totally different from that of a conventional electromagnetic transformer, has been proposed.
Throughout the instant disclosure, like portions or elements of the drawings are denoted by like reference numerals.
Before turning to the present invention it is deemed preferable to describe two conventional step-up type piezoelectric transformers with reference to FIGS. 1 and 2.
The piezoelectric transformer as shown in FIG. 1 is disclosed in U.S. Pat. No. 2,974,296 granted to C. A. Rosen (Prior Art 1).
As shown in FIG. 1, a long relatively thin rectangular body 10 of piezoelectric material is divided into two regions 12 and 14 each of which extends in a longitudinal direction of the body 10 a distance equal to one half of the length thereof. The region 12 is called a driving region while the region 14 is called a driven region (or power generating region).
A pair of electrodes 16a and 16b is applied to upper and lower surfaces of the driving region 12. On the other hand, the driven region 14 has an electrode 18 applied to the end of the body 10 remote from the region 12. The driving region 12 is polarized in the thickness direction of the body 10 as indicated by arrows 13 while the driven region 14 is polarized in the longitudinal direction of the body 10 as indicated by arrows 15.
A connection wire 20a has one end thereof fixedly coupled to a junction 21 provided at the center of the electrode 16a. Similarly, a connection wire 20b has one end thereof fixedly coupled to a junction (not shown) provided at the center of the electrode 16b. On the other hand, a connection wire 24a is provided between a junction 25 and an output terminal 26a. Further, a connection wire 24b has one end thereof coupled to an output terminal 26b and the other end thereof coupled to the wire 20b.
If an input voltage Vin applied to the pair of input terminals 22a and 22b has a frequency equal to that of the longitudinal resonant mechanical vibration of the body 10, the body 10 is caused to resonantly vibrate. Thus, a high AC (alternating current) voltage Vout is obtained between the output terminals 26a and 26b. The frequency of the output voltage Vout is identical to that of the input voltage Vin.
The piezoelectric transformer of Prior Art 1 shown in FIG. 1, however, has encountered the drawback that all the junctions of the transformer are not provided at resonance nodes of the body 10. Therefore, a junction which is not provided at a resonance node, is apt to be detached from the corresponding electrode due to mechanical vibration.
FIG. 2 shows another conventional piezoelectric transformer which is disclosed in a Japanese paper entitled "Electronic Luminescence" issued March 1971, page 65 (Prior Art 2).
The transformer of FIG. 2 is similar to that of FIG. 1 except for the driven region 14. That is, the driven region 14 of FIG. 2 is polarized transversely of the body 10 as indicated by arrows and provided with two electrodes 18a and 18b at the opposite sides in a plane parallel to the longitudinal direction of the body 10. Two output wires 24a' and 24b' are respectively coupled to the electrodes 18a and 18b via corresponding junctions 23a and 23b. It is known in the art that in order to generate a high output voltage, it is necessary to reduce the thickness of the body to a considerable extent (such as less than 1 mm). Therefore, the arrangement of FIG. 2 has encountered the drawback that it is practically difficult to secure the electrodes 18a and 18b to such dimensionally limited side faces. Further, the electrodes 18a and 18b are liable to deteriorate during the processes of polarization and junction application to the electrodes.